In image forming machines such as electrographic or electrostatic copying machines, laser-beam printers, and facsimiles, a developer is used for visualizing electrostatic latent images formed on a photoreceptor. As the developer, a toner composed of colored polymer particles containing at least a binder resin and a colorant is used.
Toners are roughly divided into pulverized toners obtained by a pulverizing method and polymerized toners obtained by a polymerization method.
In the pulverizing method, a pulverized toner is obtained by melting and kneading a thermoplastic resin together with additive components such as a colorant, a charge control agent, and a release agent, and pulverizing and classifying the mixture.
In the polymerization method, a polymerized toner is obtained by polymerizing a polymerizable monomer composition containing polymerizable monomers, a colorant, and other additive components in an aqueous medium, and coagulating the composition as required, to obtain a polymerized toner as colored polymer particles. In the polymerized toner, the polymer component formed by the polymerizable monomer becomes a binder resin.
A toner is required to form delicate and high-density images of excellent image quality, not to deteriorate the image quality even by change in ambient conditions such as temperature and humidity, and to be able to perform continuous printing and continuous copying. The toner is also demanded to improve the fixation properties such as lowered fixing temperature without lowering shelf stability to satisfy the requirements of energy saving, and color printing and copying. Furthermore, as the requirement level for high resolution and high image quality is elevated, the tendency to reduce the particle size of the toner has been enhanced.
Since the particle size distribution of the pulverized toner obtained by the pulverizing method is wide, the toner must be classified to have desired particle size distribution to obtain high-quality images.
By the polymerization method, on the other hand, a polymerized toner composed of spherical particles having sharp particle size distribution can be manufactured. In addition, according the polymerization method, colored polymer particles of a core-shell structure can be formed by a polymerizable monomer for the shell in the presence of the formed colored polymer particles after a polymerizable monomer composition has been polymerized; whereby the polymerized toner of excellent fixation properties can be manufactured without deteriorating shelf stability. Furthermore, according to the polymerization method, a polymerized toner having a small particle size, for example, of the volume average particle size of 10 μm or less, or even 3 to 8 μm can be easily manufactured.
However, even by the polymerization method, the formation of fine particles cannot be prevented completely, and especially in the case of the polymerized toner of a small particle size, fine particles is easily formed causing fog and image quality lowering. Therefore, the removal of fine particles by classification has become an essential technical problem for raising the resolution of the polymerized toner, and for obtaining fine and high-quality images.
However, in not only pulverized toners, but also polymerized toners, as the particle sizes thereof are reduced, the coagulating properties tend to intensify due to the action of intermolecular force. As a result, the dispersibility or fluidity of toners of small particle sizes is deteriorated.
On the other hand, the classification of the toner is generally performed by the method for mechanically classifying the material colored polymer particles using various classifiers such as airflow classifiers and mechanical centrifugal wind classifiers. However, when colored polymer particles having large coagulation properties is supplied to a classifier, since the colored polymer particles are difficult to be dispersed to the state close to primary particles in the classifier, the highly accurate classification becomes difficult, and the toner having desired particle size distribution cannot be obtained efficiently at a high yield.
There has been known an airflow classifier equipped with a material supply nozzle having an opening in the classifying zone to inject the powder material in the material supply nozzle to the classifying zone by the air flowing in the material supply nozzle, and to classify the powder material into at least coarse particles and fine particles by the inertial force of the particles of the powder material in the injected airflow and the Coanda effect (Japanese Patent Laid-Open No. 2000-42494). This reference proposes the methods to lower the liquid cross-linking force of the powder material in the airflow when the toner is classified using the airflow classifier (also referred to as “elbow-jet classifier”) by lowering the moisture content in the airflow by controlling humidity; and to raise the classification accuracy by raising the fluidity and dispersibility of the powder material. However, the use of the airflow classifier is difficult to efficiently remove fine particles from the toner of small particle sizes even if the fluidity and dispersibility of the powder material is raised.
There has also been known a classifier having a rotatably borne drive shaft and a classifying rotor mounted to the drive shaft, and classifying powder or granules by coaxially rotating the classifying rotor synchronizing the rotation of the drive shaft (Japanese Patent Laid-Open No. 2001-104888). This classifier is a “mechanical centrifugal air classifier” for classification by the balance of the centrifugal force by the vane provided in the rotating rotor with the centripetal force by the suction of the fan. This reference proposes to carry out the surface treatment for preventing the adhesion of the powder or granules to the classifying rotor. However, even the use of this classifier is difficult to efficiently remove fine particles from the toner having a small particle size and having strong coagulation properties.
Furthermore, there has been known a classifier for classifying pulverized powder material having a plurality of vane-wheel classifying rotors on the same axis in a casing, wherein the rotation speed of each classifying rotor can be set to be the same or different from each other, and performs a plurality of stages simultaneously corresponding to each classifying rotor (Japanese Patent Laid-Open No. 2001-293438) This reference describes that the use of this classifier avoids the pulverization of powder materials, the powder material is optimally dispersed between the classifying zones, and the mass present in some cases, and the classification can be performed within an obvious particle size range. However, even the use of this classifier is difficult to efficiently remove fine particles from the toner having a small particle size and having strong coagulation properties.